Processes for the production of nitrogen and oxygen

ABSTRACT

The present invention relates to a process and an installation for obtaining oxygen and nitrogen, in gaseous form, wherein a part of the reheated pure gaseous nitrogen originating from the separation stage is compressed, cooled and condensed by exchange with a liquid natural gas being revaporised, expanded, a part of the expanded liquid fraction being at least utilized for ensuring the reflux of the said separation stage and the vaporised fraction being reunited with the nitrogen compressed before the said compression wherein liquid nitrogen and gaseous nitrogen and oxygen, are obtained at different pressure levels, the said part of the reheated pure gaseous nitrogen reunited with a part of this gaseous nitrogen extracted before the end of reheating, after condensation and before its expansion, is subcooled counter-current flow of the fraction vaporised during the said expansion after being recondensed by exchange with the pure oxygen liquid fraction obtained at the bottom of the separation and reunited after subcooling with nitrogen at intermediate pressure.

United States Patent Perrotin et a1. [45] J 1975 [54] PROCESSES FOR THE PRODUCTION OF 3,339,370 9/1967 Streich 62/40 NITROGEN AND OXYGEN 3,416,323 12/1968 Hemlk 62/41 [75] Inventors: Guy Perrotin, Sucy:en-Brie; Primary Examiner Norman Yudkoff Jean-Pierre Anselmlm, Paris, both Assistant Examiner l:rank Sever of France Attorney, Agent, or FirmYoung & Thompson [73] Assignee: LAir Liquide, Societe Anonyme pour IEtude et lExploitation des [57] ABSTRACT pmcedes Georges Claude Pans The present invention relates to a process and an in- France stallation for obtaining oxygen and nitrogen, in gase- 22 Filed; A 23, 1973 ous form, wherein a part of the reheated pure gaseous nitrogen originating from the separation stage is com- [21] Appl' 390343 pressed, cooled and condensed by exchange with a liq- Related U S A li i D uid natural gas being revaporised, expanded, apart of [63] Continuation of Sen 69591, Sept 4 1970, the expanded liquid fraction being atleast utilized for ensuring the reflux of the said separation sta e and the abandoned. g

vaporised fraction being reunlted with the nitrogen [30] Foreign Application Priority Data Compressed before the said compression wherein liqse t 10 1969 France 69 30785 uid nitrogen and gaseous nitrogen and oxygen, are obp tained at different pressure levels, the said part of the I u 2 26 reheated pure gaseous nitrogen reunited with a part of [52] U S Cl 62/30 62/39 this gaseous nitrogen extracted before the end of re- 1 Int Cl Fzsj 3/02 heating, after condensation and before its expansion, 58] Fie'ld 40 13 is subcooled counter-current flow of the fraction vaporised during the said expansion after being recondensed by exchange with the pure oxygen liquid fraction obtained at the bottom of the separation and re- [56] References Clted united after subcooling with nitrogen at intermediate UNITED STATES PATENTS pressure. 3,058,314 /1962 Gardner 62/40 3,183,677 5/1965 Tafreshi 62/40 4 Claims, 2 Drawmg Flgures N: our 15 bars T LN; our 5 bars 23 O1 5 bars 415 26 01 40 bars 16 a4 25 28 I N1 im ur 1 bar 20 2 29 A AAA 4 30 L GM 14 11 I 13 31 15 u- A 4 A 1 1g GN Q 10 v .20 LE 0 l N1 liquide PATENTEDJUNI; 97s

SHEET By y I firm s.

PROCESSES FOR THE PRODUCTION OF NITROGEN AND OXYGEN This is a continuation of application Ser. No. ($9.5M filed Sept. 4, l970. now abandoned.

The present invention relates to a process and an installation for obtaining oxygen and a high proportion of nitrogen, on th e one'hand in liquid phase, and on the other hand in gaseous phase at one or more pressure levels, starting with air and a source of available refrigeration, for example. a liquefied natural gas being revaporised.

The simultaneous production of pure oxygen and a high proportion of pure nitrogen in liquid or gaseous phase under pressure can be obtained by means of different known processes and installations.

Hence, it is possible to operate in two steps. by compression of the air to a pressure of 6 bars, for example, purification of the air, cooling, partial condensation and distillation in a double column, of which the low pressure is slightly higher than the atmopsheric pressure. However, the gases obtained in this way are for the most part produced at low pressure; in addition, the quantity of gas directly produced under pressure or in liquid form is in all cases extremely limited. The gases at low pressure have to be recomp'ressed, in fact, to a high pressure and those intended to be obtained in liquid phase have to be treated in a liquefaction apparatus.

It is also possible to operate directly at high or medium pressure; the cold-producing power necessary is then assured either by the expansion with external work of a part of the previously treated air, compressed at very high pressure (100 to 200 bars), or from a refrigeration cycle with external work, treating the purified air, which is recycled, or the nitrogen.

The known processes have not proven to be completely satisfactory in the case where it is desired to produce oxygen and a high proportion of nitrogen in gaseous form and under pressure and/or in liquid phase. They do in fact require a relatively high consump tion of energy and considerable investment costs.

The present invention permits these disadvantages to be overcome and in addition loads to certain supplementary advantages, such as simplicity in operation and flexibility, provided that an external source of refrigeration is available.

This invention does in effect make it possible to limit to the minimum the compression of gases, or carrying out these compression operations at low temperature as often as possible, to reduce the number and the total installed power and to limit the machines, of considerably the height of the exchangers (because of the treatment of the gases under pressure or by liquefactionvaporisation in the main exchange line).

This invention also makes it possible to simplify the operation of an installation for the production of oxygen and a high proportion of nitrogen in gaseous form and under pressure and/or in liquid form, because such aninstallation requires only a small number of compressors and a single turbine, these machines generally being of the centrifugal type.

Furthermore, the versatility of such an installation is such that the various products can be obtained with very variable quantities of external refrigeration, which can if necessary be reduced to zero. Moreover, the proportions between the various products can vary' within very wide limits.

LII

- The present invention has for its object a process for obtaining oxygen and nitrogen. possibly on the one hand in liquid phase and, on the other hand, in gaseous phase. wherein a part of the reheated pure gaseous nitrogen originating from the separation stage is compressed, cooled and condensed by exchange with a liquid natural gas being revaporised, expanded, a part of the expanded liquid fraction being at least utilised for ensuring the reflux of the said separation stage and the vaporised fraction being reunited with the nitrogen compressed before the said compression wherein liquid nitrogen and gaseous nitrogen and oxygen, are ob tained at different pressure levels, the said part of the reheated pure gaseous nitrogen reunited with part of this gaseous nitrogen extracted before the end of reheating, after condensation and before its expansion, is subcooled by counter-current flow of the fraction vaporised during the said expansion, possibly after being recondensed by exchange with the pure oxygen liquid fraction obtained at the bottom of the separation and reunited after subcooling with nitrogen at intermediate pressure.

According to one embodiment of the present invention, at least a part of the liquefied fraction thus expanded is at least partly used for obtaining a fraction of expanded liquid pure nitrogen, this liquid fraction being subcooled beforehand in counter-current with a fraction vaporised during the said subsequent expansion.

According to another embodiment of the present invention, a part of the pure liquid oxygen originating from the said separation is compressed, heated and vaporised, the other part being subcooled in countercurrent with a vaporised fraction during the subsequent expansion and partially obtained expanded to the liquid state.

According to another embodiment of the invention, the compression of the heated gaseous nitrogen is effected in a single step or in several separate steps, and preferably in two steps by passing over at an intermediate pressure.

According to another embodiment of the invention, the gaseous nitrogen, compressed, cooled and condensed by exchange with a liquid natural gas being revaporised, is subcooled in at least two steps before two successive expansion; the fraction vaporised during the first expansion and having effected the first subcooling is reunited with a part of the nitrogen compressed at intermediate pressure, then liquefied, preferably by exchange with the oxygen-enriched liquid fraction obtained in the said separation, then subcooled, preferably by exchange, at least with the pure gaseous nitrogen originating from the separation, and finally reunited with the liquid fraction, obtained after the first expansion, before the latter fraction is subjected to the said second expansion.

According'to one embodiment of the invention. the said separation is effected by distillation at a single pressure or by distillation at two different pressures.

According to another embodiment of the present invention, prior to the separation by distillation, the air is compressed to a first level, purified, then cooled by exchange with a fraction of natural gas and finally compressed to a higher pressure level.

Accordin'g'to one embodiment of the invention, one or more of the following components are obtained: liquid nitrogen and oxygen in pure form at relatively low pressure (for example atmospheric pressure),.gaseous nitrogen and oxygen in pure form at medium pressure (for example, of the order of 5 bars). gaseousnitrogen and oxygen in pure form at possibly different high pres sures (such as at pressures of respectively 15 and 40 bars), by expansion of a part of the liquid pure oxygen originating from the separation, after subcooling by the gaseous fractions due to the expansion: by expansion of a part of the pure liquid nitrogen originating, in gaseous form, from the separation, and having been compressed in one or more steps to the said high pressure, then cooled and liquefied by exchange with a liquid natural gas by reyaporisation and expanded in one or more steps after one or more subcooling operations with the gaseous fractions obtained during the said expansion operations; by vaporisation and heating of a part of the liquid pure oxygen originating from the separation; by heating of a part of the pure gaseous nitrogen originating from the separation; by compression of a part of the liquid pure oxygen originating from the separation, followed by vaporisatiion and reheating; by compression of a part of the gaseous pure nitrogen originating from the separation, previously at least in part completely reheated.

Other objects and advantages of the present invention will become apparent from reading the following description and the accompanying Figures, given simply by way of example.

FIG. 1 shows one embodiment of an installation ac cording to the present invention, comprising a double distillation column and a single compression stage of the gaseous pure nitrogen.

FIG. 2 represents another embodiment of an installation according to the present invention, comprising a distillation column under a single pressure and two stages for compressing gaseous pure nitrogen.

FIG. 1 illustrates one application of the process, in the case where it is desired to produce simultaneously gaseous oxygen at 5 and 40 bars, gaseous nitrogen at 5 and 15 bars, liquid oxygen and liquid nitrogen, making use of the refrigeration of a liquefied natural gas undergoing revaporisation.

The air to be treated enters the unit at 1, after being filtered. It is compressed at 6 bars in the first stages of the isothermal compressor 2, then cleansed from water and carbon dioxide by absorption of these impurities in the purification stage 3.

It is then cooled to about lOOC in the exchanger 4 through which passes a fraction of the natural gas, then reintroduced by way of the line 5 into the upper stages of the compressor 2, which it leaves at the pressure of about 15 bars. It is then introduced by way of the line 6 into the exchangers 7, in which it is cooled and partially liquefied.

It then travels by way of the line 8 into the medium pressure column 9, where it experiences a first rectification.

Three liquid fractions are extracted from this column. a fraction rich in oxygen (line 10), a pure nitrogen fraction (line 11) and a fraction of intermediate composition (line 12).

These three fractions are cooled in the exchangers 13 in counter-current with departing products, before introduction into the column 14.

At the base of the latter, there are extracted:

gaseous pure oxygen at 5 bars (line 15) then heated in the exchange line 7;

liquid oxygen (line 16 a fraction of the latter is compressed at bars in the pumps 17, then vaporised and reheated in the exchangers 7',

a second fraction is directed towards the subcooler 19, in which it is cooled in counter-current with the vaporised fraction in the expansion which follows this exchanger.

The vaporised fraction is returned to the impure" nitrogen flow (line 20).

The impure nitrogen is extracted at the intermediate position in the column 14 and this nitrogen is reheated in the exchangers l3 and some of the exchangers 7 before being expanded in the turbine 21 and then reheated in the line 20.

At the top, the pure nitrogen fraction is extracted by way of the line 22; it is reheated in the exchangers 13 and then leaves under a pressure of 5 bars (line 23). A part of this fraction is drawn off at line 24 from the exchangers 7 before complete reheating, and the mixed with a part of the nitrogen obtained at the hot end of the exchangers 7.

After being mixed with cold nitrogen at 5 bars from the line 25 (see below), this fraction is compressed in the compressor 26 operating at low temperature.

During the compression, in the compressor 26, a nitrogen fraction at 15 bars is drawn off and delivered directly for being used (line 27).

The remainder of the nitrogen is compressed at 40 bars, then cooled and condensed in the exchanger 28 in counter-current with the natural gas at bars which is being vaporised.

This liquefied nitrogen is super-cooled in the exchanger 29 in countercurrent with the fraction vapo rised during the expansion at 5 bars, which follows this exchanger. The vaporised fraction is returned to the intake of the compressor 26 through the line 25.

The liquid nitrogen at 5 bars, obtained in the separator 30, has two destinations:

one part is directed via the line 31 towards the subcooler 32 and then expanded; the liquid fraction thus collected in the separator 33 is supplied for use.

The fraction vaporised during the expansion rejoins the impure nitrogen by way of the line 20.

the other part is sent via the line 34 to return to the column 14.

FIG. 2 illustrates a modification of the installation shown in FIG. 1, which only differs from the latter as regards the following main points;

the air is only compressed to about 5 bars in a single compressor, then purified;

after cooling in the exchangers 7, the air, partly liquefied, is directly introduced into the column 14;

the part of the cold production which was assured by the compression of the treated air at 5 to 15 bars is replaced by a nitrogen cycle between 5 and l5 bars. The nitrogen at 15 bars is drawn off on the one hand at the delivery end of the first stage of the compressor 26 and on the other hand in the separator 30 at 15 bars. After cooling in the exchangers 7, it is liquefied in the oxygen bath at the bottom of the column 14 and subcooled in 13. It is then removed, after vaporisation in the column 14, by the lines 23 and 24 and also by the line 25.

It is obvious that the invention is not limited to the embodiments which have been described, and that it is capable of numerous modifications available to the person skilled in the art. without thereby departing from the scope of the invention. Thus, the nitrogen cycle according to FIG. 2 can be used in the case of FIG. 1. In this case, the nitrogen at bars is introduced into the column 9 and liquefied at the top in contact with liquid oxygen. After being subcooled in 13, it can also be introduced after expansion at 5 bars into the column 14 and hence participate in the rectification. p 1

In particular, the energy necessary for the compression of the pure gaseous nitrogen can be at least partly produced by expansion of a fluid flowing through an auxiliary cycle, firstly undergoing cooling and condensation by heat exchange with the said source of available refrigeration, compression by a pump and vaporisation. This fluid flowing through the auxiliary cycle can be a single constituent, such as ethylene ethane, propane; it can also be composed of at least two constituents, such as an ethane-propane-butane mixture. The expansion in the turbine can be accompanied by a partial liquefaction. The said vaporisation, following the compression of the fluid flowing through the auxiliary cycle, is obtained by heat exchange with a flow at a higher temperature and particularly by means of residual vapours drawn off from the vapour cycle generation energy for the compression of the air prior to the treatment for separation of the air.

What we claim is:

l. A process for separating air into oxygen and nitrogen, while vaporizing liquefied natural gas, comprising:

a. cooling and at least partially liquefying a compressed feed stream of air, by heat exchange with at least a part of oxygen and nitrogen separated from air;

b. separating said liquefied air feed stream in at least a rectification zone under a pressure substantially above atmospheric pressure, into oxygen and nitrogen, whereby at least said nitrogen is obtained under said pressure substantially above atmospheric;

c. work expanding at least a part of said nitrogen in gaseous phase to around atmospheric pressure, and heating said expanded part in heat exchange with the compressed air feed stream;

d. compressing a stream of said oxygen in liquid phase above said pressure substantially above atmospheric;

e. vaporizing and heating said compressed oxygen stream in heat exchange with at least said air feed stream;

f. heating another part of said nitrogen in gaseous phase to a temperature below ambient temperature;

g. compressing a stream of said heated nitrogen;

h. at least partially condensing said compressed nitrogen stream in countercurrent heat exchange with liquefied natural gas being vaporized, wherein said compressed stream of nitrogen is liquefied under a lower pressure than the pressure at which liquefied natural gas is vaporized in heat exchange with said compressed stream;

i. expanding at least a part of said liquefied nitrogen stream to said pressure substantially above atmospheric and returning said part to the top of said rectification zone as reflux.

2. A process for separating air into oxygen and nitro- 6 gen, while vaporizing liquefied natural gas. comprising:

a. cooling and at least partially liquefying a compressed feed stream of air, by heat exchange with at least a part of oxygen and nitrogen separated from air;

b. separating said liquefied air feed stream in at least a-rectification zone under a pressure substantially above atmospheric pressure, into oxygen and nitrogen, whereby at least said nitrogen is obtained under said pressure substantially above atmospheric;

c. work expanding at least a part of said nitrogen in gaseous phase to around atmospheric pressure, and heating said expanded part in heat exchange with the compressed air feed stream;

(1. heating another part of said nitrogen in gaseous phase to a temperature below ambient temperature.

e. compressing a stream of said heated nitrogen;

f. at least partially condensing said compressed nitrogen stream in countercurrent heat exchange with liquefied natural gas being vaporized, wherein said compressed stream of nitrogen is liquefied under a lower pressure than the pressure at which liquefied natural gas is vaporized in heat exchange with said compressed stream;

g. expanding at least a part of said liquefied nitrogen stream to said pressure substantially above atmospheric and returning said part to the top of said rectification zone as reflux;

h. withdrawing from said top of said rectification zone in vapor phase a substantially pure nitrogen product.

3. A process for separating air into oxygen and nitrogen, while vaporizing liquefied natural gas, comprising:

a. cooling and at least partially liquefying a compressed feed stream of air, by heat exchange with at least a part of oxygen and nitrogen separated from air;

b. separating said liquefied air feed stream in at least a rectification zone under a pressure substantially above atmospheric pressure, into oxygen and nitrogen, whereby at least said nitrogen is obtained under said pressure substantially above atmospheric;

c. compressing a stream of said oxygen in liquid phase above said pressure substantially above atmospheric;

d. vaporizing and heating said compressed oxygen stream in heat exchange with at least said air feed stream;

e. heating a part of said nitrogen in gaseous phase to a temperature below ambient temperature;

f. compressing a stream of said heated nitrogen;

g. at least partially condensing said compressed nitrogen stream in countercurrent heat exchange with liquefied natural gas being vaporized, wherein said compressed stream of nitrogen is liquefied under a lower pressure than the pressure at which liquefied natural gas is vaporized in heat exchange with said compressed stream;

h. expanding at least a part of said liquefied nitrogen stream to said pressure substantially above atmospheric and returning said part to the top of said rectification zone as reflux;

i. withdrawing from said top of said rectification zone in vapor phase a substantially pure nitrogen product.

4. A process for separating air into oxygen and nitrogen, while vaporizing liquefied natural gas. comprising:

a. cooling and at least partially liquefying a compressed feed stream of air, by heat exchange with at least a part of oxygen and nitrogen separated from air;

b. separating said liquefied air feed stream in at least a rectification zone under a pressure substantially above atmospheric pressure, into oxygen and nitrogen, whereby at least said nitrogen is obtained under said pressure substantially above atmospheric;

c. heating at least a part of said nitrogen in gaseous phase to a temperature below ambient temperature;

d. compressing a stream of said heated nitrogen;

e. at least partially condensing said compressed nitrogen stream in countercurrent heat exchange with liquefied natural gas being vaporized, wherein said compressed stream of nitrogen is liquefied under a lower pressure than the pressure at which liquefied natural gas is vaporized in heat exchange with said compressed stream;

f. expanding at least a part of said liquefied nitrogen stream to said pressure substantially above atmospheric and returning said part to the top of said rectification zone as reflux;

g. withdrawing from said top of said rectificatiion zone in vapor phase a substantially pure nitrogen product. 

1. A process for separating air into oxygen and nitrogen, while vaporizing liquefied natural gas, comprising: a. cooling and at least partially liquefying a compressed feed stream of air, by heat exchange with at least a part of oxygen and nitrogen separated from air; b. separating said liquefied air feed stream in at least a rectification zone under a pressure substantially above atmospheric pressure, into oxygen and nitrogen, whereby at least said nitrogen is obtained under said pressure substantially above atmospheric; c. work expanding at least a part of said nitrogen in gaseous phase to around atmospheric pressure, and heating said expanded part in heat exchange with the compressed air feed stream; d. compressing a stream of said oxygen in liquid phase above said pressure substantially above atmospheric; e. vaporizing and heating said compressed oxygen stream in heat exchange with at least said air feed stream; f. heating another part of said nitrogen in gaseous phase to a temperature below ambient temperature; g. compressing a stream of said heated nitrogen; h. at least partially condensing said compressed nitrogen stream in countercurrent heat exchange with liquefied natural gas being vaporized, wherein said compressed stream of nitrogen is liquefied under a lower pressure than the pressure at which liquefied natural gas is vaporized in heat exchange with said compressed stream; i. expanding at least a part of said liquefied nitrogen stream to said pressure substantially above atmospheric and returning said part to the top of said rectification zone as reflux.
 1. A PROCESS FOR SEPARATING AIR INTO OXYGEN AND NITROGEN, WHILE VAPORIZING LIQUEFIED NATURAL GAS, COMPRISING: A COOLING AND AT LEAST PARTIALLY LIQUEFYING A COMPRESSED FEED STREAM OF AIR, BY HEAT EXCHANGE WITH AT LEAST A PART OF OXYGEN AND NITROGEN SEPARATED FROM AIR, B. SEPARATING SAID LIQUEFIED AIR FEED STREAM IN AT LEAST A RECTIFICATION ZONE UNDER A PRESSURE SUBSTANTIALLY ABOVE ATMOSPHERIC PRESSURE, INTO OXYGEN AND NITROGEN, WHEREBY AT LEAST SAID NITROGEN IS OBTAINED UNDER SAID PRESSURE SUBSTANTIALLY ABOVE ATMOSPHERIC, C. WORK EXPANDING AT LEAST A PART OF SAID NITROGEN IN GASEOUS PHASE TO AROUND ATMOSPHERIC PRESSURE, AND HEATING SAID EXPANDED PART IN HEAT EXCHANGE WITH THE COMPRESSED AIR FEED STREAM, D. COMPRESSURE SUBSTANTIALLY ABOVE ATOMOSPHERIC, ABOVE SAID PRESSURE SUBSTANTIALLY ABOVE ATMOSPHERIC, E. VAPORIZING AND HEATING SAID COMPRESSED OXYGEN STREAM IN HEAT EXCHANGE WITH AT LEAST SAID AIR FEED STREAM, F. HEATING ANOTHER PART OF SAID NITROGEN IN GASEOUS PHASE TO A TEMPERATURE BELOW AMBIENT TEMPERATURE, G. COMPRESSING A STREAM OF SAID HEATED NITROGEN, H. AT LEAST PARTIALLY CONDENSING SAID COMPRESSED NITROGEN STREAM IN COUNTERCURRENT HEAT EXCHANGE WITH LIQUEFIED NATURAL GAS BEING VAPORIZED, WHEREIN SAID COMPRESSED STREAM OF NITROGEN IS LIQUEFIED UNDER A LOWER PRESSURE THAN THE PRESSURE AT WHICH LIQUEFIED NATURAL GAS IS VAPORIZED IN HEAT EXCHANGE WITH SAID COMPRESSED STREAM, I. EXPANDING AT LEAST A PART OF SAID LIQUEFIED NITROGEN STREAM TO SAID PRESSURE SUBSTANTIALLY ABOVE ATMOSPHERIC AND RETURNING SAID PART TO THE TOP OF SAID RECTIFICATION ZONE AS REFLUX.
 2. A process for separating air into oxygen and nitrogen, while vaporizing liquefied natural gas, comprising: a. cooling and at least partially liquefying a compressed feed stream of air, by heat exchange with at least a part of oxygen and nitrogen separated from air; b. separating said liquefied air feed stream in at least a rectification zone under a pressure substantially above atmospheric pressure, into oxygen and nitrogen, whereby at least said nitrogen is obtained under said pressure substantially above atmospheric; c. work expanding at least a part of said nitrogen in gaseous phase to around atmospheric pressure, and heating said expanded part in heat exchange with the compressed air feed stream; d. heating another part of said nitrogen in gaseous phase to a temperature below ambient temperature. e. compressing a stream of said heated nitrogen; f. at least partially condensing said compressed nitrogen stream in countercurrent heat exchange with liquefied natural gas being vaporized, wherein said compressed stream of nitrogen is liquefied under a lower pressure than the pressure at which liquefied natural gas is vaporized in heat exchange with said compressed stream; g. expanding at least a part of said liquefied nitrogen stream to said pressure substantially above atmospheric and returning said part to the top of said rectification zone as reflux; h. withdrawing from said top of said rectification zone in vapor phase a substantially pure nitrogen product.
 3. A process for separating air into oxygen and nitrogen, while vaporizing liquefied natural gas, comprising: a. cooling and at least partially liquefying a compressed feed stream of air, by heat exchange with at least a part of oxygen and nitrogen separated from air; b. separating said liquefied air feed stream in at least a rectification zone under a pressure substantially above atmospheric pressure, into oxygen and nitrogen, whereby at least said nitrogen is obtained under said pressure substantially above atmospheric; c. compressing a stream of said oxygen in liquid phase above said pressure substantially above atmospheric; d. vaporizing and heating said compressed oxygen stream in heat exchange with at least said air feed stream; e. heating a part of said nitrogen in gaseous phase to a temperature below ambient temperature; f. compressing a stream of said heated nitrogen; g. at least partially condensing said compressed nitrogen stream in countercurrent heat exchange with liquefied natural gas being vaporized, wherein said compressed stream of nitrogen is liquefied under a lower pressure than the pressure at which liquefied natural gas is vaporized in heat exchange with said compressed stream; h. expanding at least a part of said liquefied nitrogen stream to said pressure substantially above atmospheric and returning said part to the top of said rectification zone as reflux; i. withdrawing from said top of said rectification zone in vapor phase a substantially pure nitrogen product. 